Components cast from nickel-based superalloys are known to exhibit excellent mechanical tensile, fatigue strength and creep resistance at high temperatures. Such components are also required to exhibit good surface stability, and particularly oxidation and corrosion resistance. Nickel-based superalloys are employed in the casting of jet engine turbine blades and vanes for commercial and military aircraft. They are also employed in gas turbines used for utility, industrial and marine power generation.
Over the past thirty five years, the high temperature performance capability of cast superalloys has been improved very substantially due to the development of directionally solidified and single crystal casting technology and alloys such as those manufactured by Cannon Muskegon Corporation under the designation CMSX-4® and those alloys developed by GE (René N-5 alloy) and PWA (PWA 1484 alloy).
Single crystal (SX) CMSX-4® alloy castings have a 70% volume fraction of fine gamma prime (γ′) precipitate strengthening phase after very high temperature heat treatment solutioning, without incipient melting. Such casting components exhibit exceptional resistance to creep under high temperature and stress, particularly in that part of the creep-rupture curve representing one percent or less elongation, while also providing good oxidation resistance. The CMSX-4® alloys, described in U.S. Pat. Nos. 4,643,782 and 5,443,789, generally represent the state of the art. CMSX-4® alloy has been successfully used in numerous aviation and industrial and marine gas turbine applications since 1991. Close to ten million pounds (1300 heats) of CMSX-4® have been manufactured to date with total turbine engine experience of over 120 million hours. An improved version of CMSX-4®, which is pre-alloyed with lanthanum and yttrium and consists of low sulfur content of about 1 ppm (by weight), has good alloy cleanliness in terms of stable oxide inclusions, as represented by 1-2 ppm oxygen content over multiple heats. Rare earth element additions, such as lanthanum and yttrium have been beneficial to alloy oxidation performance by tying up deleterious sulfur (S) and phosphorus (P) as very stable sulphide and phosphide phases. Improvement in bare alloy oxidation behavior to minimize blade tip degradation and improve thermal barrier coating (TBC) adherence is of particular interest. The addition of rare earth elements dramatically improves the dynamic cyclic oxidation behavior of CMSX-4®. An example of the benefits of adding lanthanum (La) and yttrium (Y) can be observed in the surface microstructure following creep-rupture testing at elevated temperature (e.g., 1050° C.). After 1389 hours of testing at 1050° C., no evidence of gamma prime depletion was observed, whereas without lanthanum and yttrium addition, significant gamma prime depletion would have been expected due to the diffusion of aluminum to the alloy surface to reform the alumina scale layer due to oxide scale spallation, principally resulting from S in the alloy. This improvement translates to a substantial increase in useful component life. Studies have shown that La+Y additions to CMSX-4® alloy give the best oxidation results compared to Y or La alone (FIG. 2).
The objectives for CMSX-4® were to provide sufficient creep-rupture and oxidation resistance while also exhibiting a heat treatment temperature range which permits heat treatment at a temperature at which all of the primary gamma prime phase goes into solution without the alloy reaching its incipient melting temperature. These improvements were achieved primarily by partial replacement of tungsten (W) with rhenium (Re), lowering of chromium (Cr) to accommodate the increased alloying with acceptable phase stability, and increasing tantalum (Ta). These modifications achieved the desired improvement in creep-resistance relative to known nickel-based superalloys (CMSX-3®) without excessively narrowing the heat treatment window (the difference between the temperature at which the primary gamma prime phase goes into solution and the temperature at which incipient melting occurs) and without introducing microstructural instability, thereby facilitating economical production of high performance castings for aviation and industrial gas turbine applications. Re dramatically slows down element diffusion at high temperatures.
Although the CMSX-4® alloy has been extremely successful commercially, providing improved performance, service life and economy, single crystal nickel-based superalloy castings capable of operating at even higher temperatures and providing even longer service life are desirable.